Field of the Invention
The present invention relates generally to drive circuits for controlling the turn-on/turn-off of power switching devices and more particularly to a magnetic couple drive circuit for coupling isolated power switching transistors to a minimum dissipation control circuit. The present inventive circuitry provides high speed turn-on/turn-off control of relatively high power switching devices while concurrently minimizing control circuit dissipation.
Transformer-coupled transistor drive circuits are well known in the art. These circuits commonly include regenerative current drive for the switching transistor(s) turn-on/turn-off, wherein the output current of the power transistor is transformer coupled back into the (base) drive circuit of the switching transistor(s). Specific examples of these prior art circuits may be found in U.S. Pat. Nos. 3,983,418, 3,986,052 and 4,123,670. This regenerative or enhanced turn-on/turn-off technique restricts the utility of those circuits in applications where the enhancement section of the circuit experiences transient or noise signals. Transients in the switched signal are generally amplified by the regenerative turn-on circuitry and applied to the switching control of the circuit, resulting in mis-triggering of the switching device(s). In contrast, the present drive circuitry utilizes the energy stored in the coupling magnetic to provide the requisite turn-on/turn-off drive, providing virtually total isolation of the switching devices from the control circuitry.
The art has recognized the aforedescribed deficiencies and circuits have been designed to isolate the power switching devices from the control drive circuitry. In general these circuits inductively couple stored energy through a magnetic between drive and switching circuitry. Examples of such circuits may be found in U.S. Pat. Nos. 3,820,008; 4,005,317; 4,177,393 and 4,087,703. These circuits are generally deficient in one or both of two aspects, either the control circuit is substantially dissipative during the quiescent or non-conductive period of the power switching device and/or the turn-on/turn-off capability of the drive circuit is restricted to specified time intervals, effectively slowing the turn-on/turn-off capability of the circuit. In most prior art circuits, overcoming one of these deficiencies typically worsens the other deficiency.
In contrast to the aforedescribed prior art circuits, the present invention teaches a high speed turn-on/turn-off drive circuit whose low power control circuitry is virtually isolated from the power switching device(s) yet provides turn-on/turn-off drive capability at substantially all times. In a quiescent state, that is to say during steady state non-conduction of the power switching device, the control circuitry is nominally dissipative, yet maintains virtually ever present ability to quickly drive the power switching devices into and from saturated conduction and non-conduction.